This invention relates to a process of producing sulfuric acid from SO.sub.3 -containing humid gases, in which the SO.sub.3 -containing humid gases are directly cooled with aqueous sulfuric acid, sulfuric acid is condensed in the gases cooled below the dew point of the sulfuric acid, and the water not required to form sulfuric acid is discharged as water vapor with the end gases.
In the so-called dry catalytic process, the SO.sub.2 content of dry gases is catalytically reacted to SO.sub.3, which is then absorbed in sulfuric acid. In the so-called wet catalytic process, the SO.sub.2 and H.sub.2 S contents of moist gases are catalytically reacted to form SO.sub.3. Because the absorption of the SO.sub.3 content in sulfuric acid is rendered difficult due to the formation of sulfuric acid mists, generally the sulfuric acid which is formed from the SO.sub.3 and water vapor is condensed out. SO.sub.2 -containing humid gases are formed, e.g., as a result of the combustion of water vapors which contain H.sub.2 S, or of the after-burning of H.sub.2 S- and SO.sub.2 -containing exhaust gases from the Claus process, or of metallurgical processes. In the contact-process plant, the H.sub.2 S- and SO.sub.2 -containing gases having an oxygen content which is sufficient for the oxidation to SO.sub.3 are reacted at suitable temperatures in contact with V.sub.2 O.sub.5 catalysts and the sulfuric acid is subsequently condensed out in a condensing plant. Surplus water vapor is discharged in the exhaust gas. Other processes may also result in SO.sub.3 -containing humid gases from which sulfuric acid must also be condensed out.
It is known from German Patent 607,216 to cool in a first stage to 280.degree.-300.degree. C. the SO.sub.3 -containing humid gas which leaves the contact process plant and then to condense out in a second stage the sulfuric acid by a cooling of the gas to about 150.degree. C. The cooling in the first stage may be accomplished by an indirect cooling with air or by a spraying of water. The condensing stage consists of packed condensation columns. The cooling resulting in the condensation must be slow and must be performed in a time of not less than 0.5 second. The condensation columns must be correspondingly large.
German Patent 641,258 describes a process in which cooling and condensation are performed in a common stage. In a trickling tower, the gases are contacted with water or dilute sulfuric acid trickling in a countercurrent and are thus cooled from 300.degree. -400.degree. C. to 100.degree. C. Sulfuric acid mists are thus formed at high rates and must be collected from the gases in a succeeding electrical gas purifier.
U.S. Pat. No. 2,199,691 describes also the cooling and condensation in a common stage, in which the gases are contacted with sulfuric acid trickling in a countercurrent. The gases are thus cooled from about 315.degree. C. to and below 110.degree. C. The sulfuric acid is withdrawn from the trickling tower at 170.degree.-230.degree. C. and is mixed with cooled sulfuric acid, which is at a temperature below 110.degree. C., to produce mixed acids which are at a temperature below 130.degree. C. The mixed acids are cooled below 110.degree. C. in a cooler, one partial stream of said cooled acid is fed to the mixing step and another partial stream to the top of the trickling tower. This process has the disadvantages that the gases must be indirectly cooled at high expense before entering the condenser and that the entire heat supplied to the acid during the cooling and condensing steps must be dissipated by an indirect cooling of the acid.
British Patent 692,109 describes a common cooling and condensing stage, in which the hot gas is quenched in an ejector to a temperature of about 60.degree. C. by a contact with cold sulfuric acid at about 20.degree. C. The resulting sulfuric acid mists must be precipitated in an electrostatic precipitator, the entire heat supplied to the acid must be dissipated by an indirect cooling at high expense, and highly concentrated acid cannnot be produced if the gas has a high water vapor content.
It is known from Opened German Specification 1,792,577 to convert the SO.sub.2 content in the exhaust gases from steam boiler plants to SO.sub.3 by a catalytic reaction, then to cool the SO.sub.3 -containing gas to temperatures of 200.degree.-140.degree. C. by an indirect heat exchange, and to collect the resulting sulfuric acid mists in a Cottrell collector at the same temperature. The practice involves a high risk of a formation of condensate during the cooling in the indirect heat exchangers and difficulties regarding the collection of the mists.
It is known from Printed German Application 1,467,157 first to cool the gases from about 275.degree. C. to 245.degree. C. in a concentrating unit, in which the gases are contacted with counter-flowing 80% sulfuric acid which trickles at a low rate and is at about 160.degree. C., and then to cool in a second stage the gases with counterflowing sulfuric acid which has a concentration of about 80% and trickles at a high rate and cools the gases sufficiently for a conndensation of the sulfuric acid out of the gases. The first stage results in an acid of 94% which is at 250.degree. C. In this process, hot sulfuric acid must be recycled by pumps and the gases must be indirectly cooled at high expense before they enter.